Electrophotographic imaging processes and techniques have been extensively described in the prior art. Generally, such processes have in common the step of imagewise exposing a photoconductive element to electromagnetic radiation to which the element is sensitive, thereby forming a latent electrostatic charge image. A variety of subsequent operations, well known in the art, are then employed to produce a permanent record of the image.
One type of photoconductive element particularly useful in electrophotography employs a composition containing a photoconductive material and an electrically insulating resinous binding material. An integrated electrophotographic element incorporating such a composition is generally produced in a multilayer type of structure by coating a layer of the above-described composition onto a support previously overcoated with a layer of an electrically conducting material. Alternatively, the above-described composition can be coated directly onto a conductive support made of metal or other suitable conductive materials.
The desired electrophotographic properties are dictated by the end use contemplated for the photoconductive element. In many such applications, it is desirable for the photoconductive element to exhibit high speed, as measured by an electrical speed or characteristic curve, a low residual potential after exposure and resistance to electrical fatigue. Various other applications specifically require that the photoconductive element be capable of high speeds with infrared radiation (about 870 to about 970 nm).
High speed "heterogeneous" or "aggregate" photoconductive systems have been developed which exhibit many of the desirable qualities mentioned above. These aggregate compositions are the subject matter of William A. Light, U.S. Pat. No. 3,615,414 issued Oct. 26, 1971, and Gramza et al., U.S. Pat. No. 3,732,180 issued May 8, 1973. These heterogeneous or aggregate photoconductive elements comprise photoconductive compositions containing a continuous polymer phase having dispersed therein co-crystalline particles composed on a pyrylium or thiopyrylium salt and a polymer. Although these elements are useful in many applications, they do not respond to infrared activating radiation.